What happened here?
 

Brand new blower motor installed with original squirrel cage. Probably has less than 20 hours on it. Started making a lot of noise so I reluctantly pulled it out and found that the center section of the squirrel cage spins freely. This is on my 70 LeMans.

I never looked closely at those things, is that center section a press fit that is now loose perhaps?

Yes.

I would tack it but the wheel is steel and this center part appears to be aluminum, definitely not steel.

Okay put it in my little 12-ton press with the appropriate size sockets and pressed it back to a non slip fit. ��
And that was after calling around and looking for one without any luck.

with it now in a non-slip fit the only other thing I might would do would be the famous JB weld. or run it till is gives up....then maybe make some serrations in the steel and re-squash your allum center

I considered using some epoxy but it seemed to crimp down really well by using the press. It will probably hang on another 50 years before it needs attention again or whenever this new motor dies.
And by the way, I was able to access the blower motor by slipping the inner fender down enough to reach it. Really really happy I did not cut the inner fender. And this is an AC car too!

With luck, the restored press-fit will be good "forever".

If it was me, I'd have drizzled a couple of drops of Loctite 271 in between the two pieces before squeezing 'em. Probably not really needed, though.